The present invention relates to the development of novel materials that can be used in a process such as high performance liquid chromatography (HPLC), liquid chromatography (LC), thin layer chromatography (TLC), capillary electro chromatography (CEC) and counter-current chromatography. The materials are composed of support materials and saccharide moieties, especially glucose moieties preferably in the form of cyclodextrins, mutually cross-linked and chemically bonded via urea linkages. The invention further relates to processes for the production of these materials and their use in separating compounds and especially resolving enantiomeric mixtures.
Generic applicability of cyclodextrins in chromatographic separation and purification processes is described at length in reviews by W. L. Hinze, Separation and purification methods, 1981, 10(2), 159-237. Y. Kawaguchi, et al., Anal. Chem., 1983, 55, 1852; D. W. Armstrong, et al., Anal. Chem., 1985, 57, 234 and S. Li, et al., Chem. Rev., 1992, 92, 1457. Chromatographic separation on chiral stationary phases (CSP) is also the most convenient analytical method for the determination of enantiomeric purity (see for example S. G. Allenmark, Chromatographic Enantioseparations: Methods and Applications, 2nd ed., Prentice Hall, N.J., 1991). In recent years, tremendous research efforts were made in bonding cyclodextrins to solid matrices, such as silica gel, via amino or amido linkages. However, these bonds are inherently unstable to hydrolysis, thus placing severe limitations on use of these materials in aqueous media. Alternative approaches for immobilizing cyclodextrin using hydrolytically more stable ether linkages (U.S. Pat. No. 4,539,399) or carbamic acid moieties (U.S. Pat. No. 5,104,547) were also investigated. However, in all these approaches, regioselective derivatisation of cyclodextrin cannot be readily effected due to the presence of multiple hydroxy moieties in the cyclodextrin starting materials. Thus, reaction may take place on the 2, 3 or 6-position of glucose moieties of cyclodextrin, which is hard to resolve.
It has been reported that derivatised cyclodextrin stationary phases for liquid chromatography show definite enantioselectivity for a variety of compounds while pristine cyclodextrin bonded stationary phases display low enantioselectivity. Enantioselectivity of the materials was generally improved by increasing the degree of derivatisation of the xe2x80x94OH groups on cyclodextrin with carbamate groups, and by increasing the surface concentration of cyclodextrin immobilized on the support materials (D. W. Armstrong et al., Anal. Chem., 1990, 62, 1610; T. Hargitai et al., J. Chromatogr., 1993, 628, 11; T. Hargitai, et al., J. Liq. Chromatogr., 1993, 16(4), 843). In order to maximize the extent of cyclodextrin derivatisation, large molar excesses of derivatising reagents under vigorous conditions were often used. However, the derivatisation processes invariably involved the prior immobilisation of underivatised cyclodextrin on the support material followed by derivatisation procedures involving solid-liquid phases. This usually results in partial derivatisation of the hydroxyl groups of the cyclodextrin and also in large, sterically encumbered cyclodextrins having a low extent of derivatisation. These methods did not give good reproducibility or uniformity of product, with the consequence that separation of enantiomers may vary from batch to batch of the obtained CD-based CSP.
Ng, et al., U.S. Pat. No. 6,017,458 describe a procedure of immobilizing perfunctionalized cyclodextrin onto the surface of a support. The patent says that the cyclodextrin is immobilized via a urethane linkage, but it is believed that this is not correct and the linkage is a urea linkage. The procedure provides an efficient method with well-defined chemical structure and very good reproducibility. However, in these examples, and in the other instances listed above, the cyclodextrins were immobilized onto the support as small molecules, which potentially limit their stability in mobile phases with high aqueous content. Although the patent mentions monoazido and diazido cyclodextrins, only monoazido cyclodextrins are used in the examples of the patent and each immobilized cyclodextrin will have only one urea linkage linking it to the support material.
Polysiloxane with cyclodextrin anchored to its side chain has been prepared and coated onto the surface of silica gel. This material exhibits interesting properties in reverse phase HPLC. (V. Schurig, et al., J. Chromatogr. A, 1996, 755, 299; V. Schurig, et al., Ger Offen DE 43 24 636 A1 (1994), V. Schurig, et al., Angew. Chem. Int. Ed. Engl., 1994, 33, 2222). However, there is no report of the cyclodextrin polymer immobilized onto a support and applied in chiral separation.
In one aspect the invention provides a conjugate comprising a support material linked to oligomers or polymers of a saccharide which linking is via urea linkages between the saccharide moieties and the support material, and wherein the oligomers or polymers of the saccharide are also cross-linked via urea linkages.
In another aspect the invention provides a process for preparing a conjugate of the invention, which process comprises: (a) reacting an oligomer or polymer of a saccharide bearing a plurality of azide groups with an amine, preferably a primary amine, a phosphine and CO2, the amine being on the surface of a support material; or (b) reacting an oligomer or polymer of a saccharide bearing a plurality of azide groups with an amine, preferably a primary amine, a phosphine and CO2, wherein the amine is an alkenylamine, subsequently hydrosilylating the alkenyl moiety of the product with a hydrosilylating agent that bears one or more readily hydrolysable groups on the silicon atom and thereafter reacting with a support member; or (c) reacting an oligomer or polymer of a saccharide bearing a plurality of azide groups with an amine, preferably a primary amine, a phosphine and CO2, wherein the amine is present in a molecule that bears a silicon atom bearing at least one readily hydrolysable group, and thereafter reacting with a support member; or (d) reacting an oligomer or polymer of a saccharide bearing a plurality of amine groups, preferably primary amine groups, with an azide group, a phosphine and CO2, the azide group being on the surface of a support material; or (e) reacting an oligomer or polymer of a saccharide bearing a plurality of amine groups, preferably primary amine groups, with an azide, a phosphine and CO2, wherein the azide is an alkenylazide, subsequently hydrosilylating the alkenyl moiety of the product with a hydrosilylating agent that bears one or more readily hydrolysable groups on the silicon atom and thereafter reacting with a support member; or (f) reacting an oligomer or polymer of a saccharide bearing a plurality of amine groups, preferably primary amine groups, with an azide, a phosphine and CO2, wherein the azide is present in a molecule that bears a silicon atom bearing at least one readily hydrolysable group, and thereafter reacting with a support member.
The present invention makes use of a Staudinger reaction, in which an azide group, an amine group, CO2 and a phosphine react to form a urea. Azide groups can be present on the oligomer or polymer of a saccharide, and amine groups can be present on the support material, or on a molecule that will subsequently be linked to the support material, so that the saccharide is linked via urea to the support material. Alternatively, amine groups can be present on the oligomer or polymer of the saccharide, and the azide groups can be present on the support material, or on a molecule that will subsequently be linked to the support material, so that again the saccharide is linked via urea to the support material. It is preferred that the azide groups are on the saccharide and the amine groups are on the support material, or on a molecule that will subsequently be linked to the support material.
Saccharides normally contain one primary hydroxyl group and several secondary hydroxyl groups. In preferred embodiments of the process of the present invention there is used an oligomer or polymer of a saccharide in which the primary hydroxyl groups of the saccharide moieties have been replaced by azide groups. For instance, glucose bears a primary hydroxyl group on the 6-carbon atom and in a preferred embodiment of the present invention there is used an oligomer or polymer of glucose in which the primary hydroxyl groups of each glucose moiety have been replaced by azide groups on each 6-carbon atom. Thus, there is used an oligomer or polymer bearing a large number of azide groups, and reaction occurs at the large number of azide groups, creating a large number of urea linkages between the oligomer or polymer and the support material, and also a large number of urea cross-linkages between the oligomer and polymer. This enhances column stability, particularly when the conjugate of the invention is used as stationary phase in chromatography and the mobile phase has high water content.
The oligomer or polymer of a saccharide can be straight-chained, or cyclic. Examples of saccharides include glucose, fructose, mannose, galactose, ribose, arabinose, xylose, lyxose, erythrose and threose, of which glucose is preferred. The subsequent description is given with respect to glucose, and particularly with respect to cyclodextrins, but it should be understood that use of oligomers and polymers of saccharides other than glucose, and glucose other than in the form of cyclodextrins, are also within the scope of the invention. For the most part, the description is given with respect to saccharides that bear azide groups and support materials that bear amine groups but it should be understood that the invention extends to use of saccharides that bear amine groups and support materials that bear azide groups.
Most preferably a cyclic oligomer is used, especially xcex1, xcex2 or xcex3 cyclodextrin composed of six, seven or eight glucose moieties, respectively. Straight-chained oligomers and polymers can be used, however, and mention is made of cellulose, amylose and pullulan as materials that can serve as the saccharide-containing oligomer or polymer, once hydroxyl groups have been replaced by azide groups. They can be used in the form of their esters, for example cellulose acetate, provided that there are sufficient free hydroxyl groups to be converted to azide groups and thereafter to participate in the reaction to form the conjugate of the invention, as described below.
To prepare the conjugate of the invention there can be used a reactant that includes glucose moieties in which hydroxy groups at the 6-positons of the glucose moieties have been replaced by azide moieties. Use is made of a reaction in which the azide-bearing glucose moieties are reacted with an amine in the presence of carbon dioxide and a reactant that will participate in the Staudinger reaction, for example a trihydrocarbylphosphine, preferably triphenylphosphine, to form a urea. This is a facile one pot reaction that proceeds in high yield. To illustrate, if the reactant that contains glucose moieties is xcex2-cyclodextrin, there is used a 6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisazido-6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisdeoxy cyclodextrin.
The amine groups may be on the surface of a support material, or the amine groups may be on a molecule that bears another functional group. That other functional group is used to join the intermediate formed by reaction between the glucose and the amine to the support material.
The amine that reacts with the azide and CO2 can be a primary or secondary amine. Primary amines are preferred and the further description refers only to primary amines.
The azide moieties can react with the amine groups on the support material and also with azide moieties on adjacent xcex2-cyclodextrin molecules, so that each cyclodextrin becomes bound not only to the support material but also to adjacent cyclodextrin molecules. Preferably all, or substantially all, of the 6-carbon atoms of the glucose moieties bear azide moieties, so the cyclodextrin moieties are securely bound to each other and to the support.
Each oligomer or polymer of glucose is linked via a plurality of urea linkages i.e., more than two linkages per oligomer or polymer, and preferably more than six linkages per oligomer or polymer. Preferably there is a urea linkage for each glucose moiety present in the oligomer or polymer. For example, if the oligomer is xcex2-cyclodextrin it is preferred that the 6-carbon atom of each of the seven glucose moieties that constitute xcex2-cyclodextrin shall bear an azide group that will participate in a urea-forming reaction, so that each cyclodextrin is linked by up to seven urea linkages.
The 6-azido glucose moieties may bear hydroxyl groups at the 2- and 3-positions, or they may bear other functional groups or protecting groups. It is preferred that either all or none of the hydroxyl groups at the 2- and 3-positions are replaced by other functional groups or protecting groups, i.e., that the glucose is perfunctionalized or is pristine, respectively. Partially functionalized materials are within the scope of the invention, however.
As groups that can replace hydroxyl groups in the 2- and 3-positions there are mentioned alkoxy groups, aryloxy groups, acyloxy groups and carbamoyloxy groups. As examples of alkoxy groups there are mentioned straight-chained and branched alkyl groups having up to about 6 carbon atoms, especially ethyl and methyl, and cycloalkyl containing 5 or 6 carbon atoms. As examples of aryloxy groups there are mentioned phenoxy and xcex1- and xcex2-naphthyloxy groups. As acyloxy groups there are mentioned alkanoyloxy groups containing up to about 6 carbon atoms, especially acetyloxy. Carbamates can be, for example, phenylcarbamoyloxy or xcex1- or xcex2-naphthylcarbamoyloxy groups.
The reactant that includes glucose moieties bearing azide groups on the 6-carbon atoms can be obtained, for example, by halogenating the glucose moieties on the 6-carbon atoms and then replacing the halogen atoms with azide groups. Thereafter, if required, the glucose can be perfunctionalized. To illustrate the procedure of B. I. Gorin, Tetrahedron Lett., 1996, 37(27), 4647; D. Alker, Tetrahedron Lett., 1994, 35(48), 9091; C. Roehoi-Stoeckel, Tetrahedron Lett., 1997 38(9), 1551; or Yoshinaga, U.S. Pat. No. 5,241,059 can be used to prepare cyclodextrins substituted by iodine or bromine atoms. (These articles are all incorporated by reference.) This is followed by reaction with an alkali metal azide, for example NaN3, LiN3 or KN3, in a polar solvent, for example dimethylformamide (DMF). If required, the other hydroxyl groups on the 2- and 3-positions can thereafter be fully derivatised to give a perfunctionalized cyclodextrin.
A 6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisazido-6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisdeoxy-2A, 2B, 2C, 2D, 2E, 2F, 2G-heptakis-O-acetyl-3A, 3B, 3C, 3D, 3E, 3F, 3G-heptakis-O-acetyl-xcex2-cyclodextrin can be prepared, for example, by reacting 6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisazido-6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisdeoxy-xcex2-cyclodextrin with an acetylating agent. The perfunctionalized cyclodextrin has been obtained in 90% yield by stirring the 6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisazido-6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisdeoxy-xcex2-cyclodextrin with acetic anhydride in pyridine at 40xc2x0 C.
Analogously, 6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisazido-6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisdeoxy-2A, 2B, 2C, 2D, 2E, 2F, 2G-heptakis-O-methylated-3A, 3B, 3C, 3D, 3E, 3F, 3G-heptakis-O-methylated-xcex2-cyclodextrin has been obtained in 65% yield by stirring with CH3I/DMF/NaH at 40xc2x0 C. Also analogously, 6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisazido-6A, 6B, 6C, 6D, 6E, 6F, 6G-heptakisdeoxy-2A, 2B, 2C, 2D, 2E, 2F, 2G-heptakis-O-phenylcarbamoyl-3A, 3B, 3C, 3D, 3E, 3F, 3G-heptakis-O-phenylcarbamoyl-xcex2-cyclodextrin has been prepared in 75% yield by stirring with phenyl isocyanate in pyridine at 80xc2x0 C. This reaction can also be carried out with xcex1- or xcex2-naphthylisocyanate in place of the phenyl isocyanate.
For those embodiments of the invention in which amine groups, rather than azide groups, are present on the saccharide, azide-bearing saccharides can be prepared as described above and the azide groups then reduced to amine groups, for example by reaction with LiAlH4. These can then be perfunctionalized if required.
The support material can be an inorganic material, for example silica gel, Al2O3, TiO2 or ZrO2, or a synthetic polymer material. In one embodiment the support material has free primary amine groups on its surface, to participate in the reaction that links the support material to the glucose moieties. The support material can be reacted with a primary amine-containing reagent to provide the required primary amine. For example silica gel can be reacted with an xcfx89-aminoalkylene-trialkoxysilane, for example 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethyl)aminopropyltriethoxysilane or 3-(2-aminoethyl)aminopropyltrimethoxysilane. Aminised silica gel can be prepared according to the method of T. Hargitai et al. J. Chromatogr., 1993, 628,11, giving the following composition as determined from elemental analysis: C, %3.25; H, %0.96; N, %0.98. The disclosure of Hargitai et al. is incorporated herein by reference.
As synthetic polymer materials that can be used as support material there are mentioned porous functional synthetic polymers, for example polymers of styrene copolymerised with another copolymerisable monomer, that bear an xe2x80x94NH2 group or an xe2x80x94N3 group, or bear a leaving group such as, for example, OH or a halogen, that can be replaced by an xe2x80x94NH2 or xe2x80x94N3 group. For instance, OH groups and halogen atoms can be replaced by N3 by reaction with an alkali metal nitride, for example sodium, lithium or potassium nitride, in a polar solvent, for example dimethylformamide. Polymers of methyl methacrylate and such a copolymerisable monomer can also be used. As suitable copolymerisable monomers there are mentioned styrenes substituted in the benzene ring by aminoalkyl groups having, say, up to 6 carbon atoms. Examples include compounds of formula: 
where n is 1, 2 or 3. Other suitable copolymerisable monomers include aminoalkyl esters of acrylic and methacrylic acid again having, say, up to 6 carbon atoms in the aminoalkyl group. Examples include compounds of formula: 
where n is 1, 2 or 3. Support materials bearing primary amine groups are commercially available.
A support material bearing azide groups can be prepared, for example, by reacting silica gel with an xcfx89-haloalkylene-trialkoxysilane, followed by reaction with an alkali metal azide to replace the halogen atom with an azide group. As examples of xcfx89-haloalkylene-trialkoxysilanes there are mentioned 3-bromopropyltrimethoxysilane, 3-bromopropyltriethoxysilane and the corresponding chloro compounds. As alkali metal azides there are mentioned sodium, potassium and lithium azides.
A support material bearing primary amine groups can be coupled directly to glucose moieties bearing azide groups to form a conjugate of the invention. In one procedure, the aminised support is stirred in a polar solvent, for example anhydrous tetrahydrofuran (THF) or anhydrous DMF, and CO2 is passed continuously through the liquid. The azide-group-containing glucose reagent is added in the polar solvent following which a reactant for the Staudinger reaction, is added in the polar solvent. The reactant for the Staudinger reaction can be a trialkylphosphine for example a tri(C1-6-alkyl)phosphine, such as trimethylphosphine, or, preferably, triphenylphosphine. Corresponding trihydrocarbyloxy phosphines can also be used, for example a tri(C1-6-alkoxy)phosphine such as trimethoxyphosphine, or triphenoxyphosphine. Passage of CO2 and stirring are continued to complete the reaction.
In another process for preparing the conjugate of the invention glucose moieties bearing azide groups are reacted with an alkenylamine, CO2 and triphenylphosphine. The alkenylamine is preferably a straight-chained xcex1-olefin with a primary amine attached to the xcfx89-carbon atom. The number of carbon atoms is not critical, but is suitably in the range from 4 to about 22, i.e., the alkenylamine has the formula:
NH2(CH2)nCHxe2x95x90CH2
where n is a number in the range 2 to 20. 6-Aminohex-1-ene is mentioned as an example. Reaction occurs between azide groups, primary amine groups and CO2 suitably under the conditions described above, resulting in compounds having urea linkages including nitrogen atoms of the azide groups and primary amines. Attached to the urea linkage is a hydrocarbyl group with an xcfx89-alkenyl moiety. The reaction can be depicted schematically as follows: 
For those embodiments of the invention in which amine groups, rather than azide groups, are present on the saccharide, use is made of an alkenylazide corresponding to the alkenylamine described above, e.g. a compound of formula:
N3(CH2)nCHxe2x95x90CH2
where n is a number in the range 2 to 20. The reaction can be depicted schematically as follows: 
The alkenylazide reactant of formula: 
can be obtained by reacting the corresponding xcfx89-haloalkene with an alkali metal azide.
The xcfx89-alkenyl moiety is then hydrosilylated by reaction with, for example, a compound of formula:
HSiR1R2R3
wherein each of R1, R2 and R3 is an alkyl group or alkoxy group of up to 6 carbon atoms, an aryl or aryloxy group wherein the aryl moiety is a phenyl or xcex1- or xcex2-naphthyl group or a halogen atom (fluorine, chlorine, bromine or iodine), provided that at least one of R1, R2 and R3 is a readily hydrolysable group such as an alkoxy or aryloxy group or a halogen atom. This compound adds to the alkenyl double bond, resulting in a group that can be schematically depicted as follows: 
This group is then reacted with a support material, for example silica gel and the readily hydrolysable group bonds to the silica. For example, if the readily hydrolysable group is an alkoxy group there will be formed an Sixe2x80x94Oxe2x80x94Si linkage to bind the cyclodextrin to the support material, with elimination of an alkanol.
In yet another process, glucose moieties bearing 6-azido groups are reacted with an aminosilane, for example a compound of formula:
NH2(CH2)mSiR1R2R3
wherein m is an integer from 1 to about 20, R1, R2 and R3 are defined above, and one or more methylene groups can be replaced by an oxygen atom or an imino group xe2x80x94NHxe2x80x94. Examples of such compounds include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethyl)aminopropyltriethoxysilane and 3-(2-aminoethyl)aminopropyltrimethoxysilane. These compounds, CO2 and triphenylphosphine are reacted under previously described conditions, to produce glucose moieties having attached to the 6-carbon atoms side chains having terminal Si atoms bearing a readily hydrolysable group. This can then be reacted with a support material, for example as described above, with formation of an Sixe2x80x94Oxe2x80x94Si linkage and elimination of an alkanol.
For those embodiments of the invention in which amine groups, rather than azide groups, are present on the saccharide there can be used an azidosilane, for example a compound of formula:
N3(CH2)mSiR1R2R3
wherein m, R1, R2 and R3 are as defined above.
It will be appreciated from the above that although the cyclodextrin and support material are linked to each other via urea linkages, it is not necessary that one nitrogen atom of the urea moiety shall be attached directly to a carbon atom of the cyclodextrin and the other nitrogen atom of the urea moiety shall be attached directly to the support; the linkage can contain other moieties in addition to the urea moiety.
After the glucose moieties have been bound to the support material it is possible to treat the support material in an xe2x80x9cend-cappingxe2x80x9d reaction in which reactive sites on the support material are protected. For instance, hydroxyl groups on silica gel can be reacted with a reactive silane such as, for example, trimethylchlorosilane or hexamethyldisilazane to block the surface hydroxyl groups.
Conjugates of the invention are particularly suitable for use in chromatography, for example high performance liquid chromatography (HPLC), liquid chromatography (LC), thin layer chromatography (TLC), capillary electro-chromatography (CEC) and counter-current chromatography. The conjugates are particularly valuable as a chiral stationary phase (CSP) for resolving enantiomeric mixtures and in determining enantiomeric purity. The conjugates of the invention permit good reproducibility of separation, even after long run times in reverse phase separations using mobile phases having a high aqueous concentration. Their utility extends beyond use in chromatography, however. They can also be used for example in electrophoresis, especially chiral electrophoretic separations.
For use in chromatography it is preferred that the support material is in the form of spherical particles whose size is preferably from about 1 xcexcm to about 20 xcexcm, more preferably about 2 xcexcm to 10 xcexcm. For use in HPLC analytical separation a particle size of about 5 xcexcm is preferred.